The present disclosure pertains to gaskets or seals for sealing concrete structures, for example, the joints of tunnel segments.
In the construction of tunnels, the contact surfaces of two abutting tunnel segments, which are generally made of precast concrete, must be sealed against the inflow or outflow of liquids, most frequently water. Such tunnels may be subway tunnels, river crossing tunnels, road and railway tunnels, cable tunnels, waste water and water supply tunnels, among other types. As a general rule, the water pressure against which the seal is provided can be in the range of between 1 and 4 bar. But, water pressures are site specific and dependent on geological conditions. Reliable sealing should be insured between tunnel segments so as to prevent or retard the ingress and egress of liquids, such as water.
The current art in the field of segmented tunnel construction utilizes two basic types of gaskets. The first of these employs glued-on gasket segments. Glued gaskets are the traditional kind of installation. In this type of gasket, the concrete tunnel segment is precast with a groove being defined in the segment. The gasket is then installed in the groove with an adhesive to keep the gasket in place. If a defect is found in a glued-in gasket, either at the manufacturing facility or in the field, the gasket needs to be removed and another gasket glued into the groove in place of the removed gasket. Also, if the groove has been damaged during the removal of a defective gasket, the groove itself must be repaired first. Such repair may be problematic in the field.
Another type of segmented tunnel construction employs a gasket having anchor legs. In other words, the gasket segment is held in place as the concrete member is cast. With this type of construction, the gasket is preinstalled in a concrete form or mold and the concrete is then poured around the gasket so that the legs or anchors of the gasket are trapped in the concrete segment being formed. After curing, the segment is demolded and removed with the anchored gasket embedded into the concrete segment. Thus, the gasket is anchored in the concrete member by anchoring legs which provide a positive locking fit. For example, the anchoring legs can have a dove-tailed configuration or be provided with a cross-section that increases towards the bottom or distal face of the anchoring leg or foot. Alternatively, or additionally, the anchoring foot can be provided with a barb or undercuts and the like.
With anchored gaskets, if the gasket is damaged, then the concrete segment may need to be discarded because there is no easy way of removing such an embedded gasket from the concrete member so as to replace it with another one. If a defect is found in the anchored gasket during inspection at the manufacturing facility, current art requires significant effort to remove the gasket from the concrete segment. Such removal may render the segment unusable. This is because the segment groove must be repaired for it to be useable again. Then, a different type of gasket can perhaps be glued into the concrete segment to make the segment useable. However, if a gasket is damaged in transit or during installation of the concrete member, for example in a tunnel, there is no quick or easy way in the field to make the concrete member or segment useable again.
Another gasket design which has been recently developed, in addition to glued and feet-anchored gaskets, is a design which it is claimed anchors a gasket bottom face into a groove in a concrete segment with thousands of fibers that are disposed on a bottom face of the gasket. Such fiber anchored gaskets are said to be easily removable from the concrete segment. However, this type of gasket has its own disadvantages, a significant one being its cost. Replacement of such a gasket would necessitate using adhesive to secure a replacement gasket in the groove of the concrete segment or member, in addition to the possibly significant effort involved in cleaning the groove which may be needed before a replacement gasket can be installed.
Another difficulty with tunnel segment gaskets is accurate fitting of the gaskets at corners of the tunnel segment. This is a significant disadvantage of known cast-in tunnel segment gaskets because the gaskets are commonly provided in the form of a frame to be cast-in adjacent to the perimeter of the concrete tunnel segment. With the known designs, problems are created because excessive rubber collects at the joint where the two linear gasket segments are connected to each other. The connection is formed by injecting or “shooting” rubber into the corner joint. A solid corner joint is thus created but at the cost of significantly restricting the ability of the gasket to move. Such movement is important for two reasons. First, an inability to move hinders the performance of the gasket in the field. A solid corner joint, which is sometimes known as a “shot joint”, allows the elastomeric or rubber to travel along the longitudinal channels defined in the adjoining gasket segments. Such a solid or filled corner joint hinders any compression or movement of the joint itself. As a result, existing solid corner tunnel segment gasket joints lead to excessive load that builds up at the corners of the concrete segments, such as tunnel segments. Such load will eventually lead to the concrete segments cracking at the corners. As a result, the gaskets will then not be securely held and leaks may well occur. Second, if the gaskets become defective, it becomes more difficult to remove them from the concrete segment due to the solid corner joints.
With regard to adhesively secured or fitted gaskets, the corner joint issue is ameliorated by allowing for higher arches in the gasket profile at the corners. But, as mentioned, adhesively secured gaskets are disadvantageous when it becomes necessary to replace a defective gasket, particularly in the field.
One known joining configuration which was said to be an improvement for cast-in-place frame-like tunnel segment gaskets is the provision of an elastomeric film that is relatively thin in nature, provided between the angled ends of two adjacent linear tunnel segment gaskets. However, this design is disadvantageous for a number of reasons. First, it is employed with cast-in-place tunnel segment gaskets where the feet of the gasket constitute anchoring legs, meaning that a defective gasket will need to be cut out of the groove defined in the concrete of the tunnel segment if it needs to be replaced. Clearly, this is difficult to do particularly in the field. Second, because only a thin film joint is provided between two linear gasket segments, this design necessitates the use of an additional strengthening element which is integral with the joint. Such a strengthening element, namely, a wedge located at the inner edge of the joint, is utilized to strengthen the joint and reduce failures in the joint from the linear gasket segments pulling apart at the joint. Thus, this known design still presents an angular corner which results in “point” forces acting on the corners of the concrete segment. In fact, pressure on the corners of the concrete segment is exacerbated by the presence of such wedges.
It would be desirable to eliminate any angular projection under the gasket acting on the corners of the concrete segment, loading the corners and making them more prone to failure. This would reduce the possibility that excessive load is placed on the corners of the concrete segment. It would also be desirable to produce a cast-in-place gasket with a softer, solid corner joint which does not require a separate strengthening element, and which corner joint does not place an excessive load on the corners of the concrete segment itself. Such corner joints would desirably connect four linear gasket segments into a generally rectangular or quadrilateral frame-like structure around the four sides of a concrete structure, such as a tunnel segment. This would create a frame-like gasket member. Moreover, it would be desirable to allow the entire frame-like gasket member to be removed, perhaps in an intact manner, from the concrete segment if some portion of the gasket becomes damaged and replace the entire frame-like gasket member either at the casting plant or at the job site without any extraordinary effort. In other words, it would be desirable to allow for a simplified removal and replacement of a damaged gasket construction in tunnel segments, such gasket constructions being generally frame-like or quadrilateral in structure, particularly in the field without the need to send the concrete tunnel segment back to the pre-cast plant for refitting with a replacement gasket.
Tunnel gasket designs are based on balancing the closure forces on the tunnel with the stress created to produce the necessary sealing capability required by particular project specifications. A constant balanced tension is required on the gaskets in order to achieve a reliable seal. Industry experts have voiced some concern regarding the potential effects of the Poisson coefficient on concrete when the closure forces allow the gasket material to flow to a point where there is a concentrated load on the corner of the last tunnel segment being installed to create the tunnel ring.
The Poisson coefficient or Poisson ratio is the negative ratio of transverse strains to axial strains on a material. When a compressive force acts on concrete, two types of strains will crop up. A first strain acts along the horizontal axis, and a second strain acts along the vertical axis. For static loads, such as in concrete, the coefficient should be about 0.20.
It would be desirable to provide a gasket which, through the function of its attachment to the concrete of the tunnel segment, precludes or minimizes the effects of the Poisson coefficient on the concrete tunnel segment by reducing the flow characteristic of the anchored gasket, versus present gasket designs used in the construction of tunnels.
It would also be advantageous to reduce labor costs that need to be incurred for field removal and replacement of gaskets because labor costs are a major component of construction project budgets. These project costs are typically cost-shared by local, state and national funding programs that are driven by tax and bond revenues.
It would therefore be desirable to provide a gasket which functions as an anchored gasket during the manufacture of concrete segments whereby an anchor element or elements act to attach or mount the gasket to the concrete segment, but which anchor element or elements allow the gasket to be removed and replaced in an economical manner if the gasket becomes damaged. It would also be desirable to provide a gasket construction which can be replaced with another gasket at the casting plant, in the storage yard, or on the job site without the need for extraordinary efforts or equipment, particularly as to field removal and replacement of the gasket. Also desirable would be the utilization of an identical replacement gasket which maintains the design criteria of the project without fear of violating any approved design parameters.